Stamping apparatus, method of stamping and stamping mold

ABSTRACT

A method of stamping for separating a workpiece part of sheet metal to obtain a workpiece, the method including a work-hardening step of generating work-hardening at vicinity of outline of the workpiece part of the sheet metal by moving a work-hardening punch having an end side facing the sheet metal toward the sheet metal and locally pressing the vicinity of the outline along the outline with a work-hardening projection provided on the end side of the work-hardening punch, and a separation step of punching the workpiece part of the sheet metal which has finished the work-hardening step with a separation punch and thereby separating the workpiece part from the sheet metal, wherein the work-hardening projection is formed to have V-shaped cross-section with both inner wall and outer wall of the work-hardening projection being inclined to pressing direction of the work-hardening punch.

BACKGROUND

The statement in this section merely provide background informationrelated to the present disclosure and do not necessarily constituteprior art. As illustrated in FIG. 1-3, a punching method is known inwhich a punch 101 is lowered against sheet metal 50 placed on a die 102,and a workpiece P (FIG. 3) is punched off by punching the sheet metal 50with the punch 101. However, the inventors recognized that, in such apunching method, roll-over A₁ was likely to be formed at the lower edgeof the periphery of the workpiece P as illustrated in FIG. 3. Theinventors also recognized that fracture surfaces A₃ and burrs A₄ werelikely to be formed above the burnished surface A₂ on the periphery ofthe workpiece P. The inventors presumed that formation of these fracturesurfaces A₃ and burrs A₄ were related to random cracks C formed in thesheet metal 50 as illustrated in FIG. 2. The cracks C started from wherethe corner of the die 102 or the corner of the punch 101 contacts withthe sheet metal 50 during the punch 101 is pressing the sheet metal 50.One approach was to remove the burrs A₄ from the workpiece P afterseparation from the sheet metal.

Alternatively, Japanese Unexamined Patent Application Publication No.59-082121 (hereinafter referred to as “Patent Document 1”) disclosed aworkpiece punching method in which a workpiece part was punched off fromsheet metal 3b in two steps. In the first step, the punch 2a having aflat end was driven into the sheet metal 3b to about half of itsthickness as illustrated in the FIG. 2 of the same document. In thesecond step, the workpiece part was separated from the sheet metal 3bwith a punch 2b and a die 1b that have larger clearance than the punch2a and die 1a used in the first step as illustrated in the FIG. 3(a),(b) of the same document.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a conventional stamping apparatuscut in a plane including center line of a punch, illustrating the statebefore the punch is lowered against sheet metal supported by a die.

FIG. 2 is a cross-sectional view of the stamping apparatus of FIG. 1 cutin the same plane as FIG. 1, illustrating the state in which the punchhas lowered from the state illustrated in FIG. 1 and is pressing thesheet metal.

FIG. 3 is a cross-sectional view of the stamping apparatus of FIG. 1 cutin the same plane as FIG. 1, illustrating the state in which the punchhas further lowered from the state illustrated in FIG. 2 and a workpiecepart of the sheet metal has been separated by the punch.

FIG. 4 is a schematic perspective view of the neighborhood of a stampingmold set in a stamping apparatus in accordance with one or moreembodiments.

FIG. 5 is a perspective view of the work-hardening punch in the stampingapparatus illustrated in FIG. 4, viewed from the end side.

FIG. 6 is a cross-sectional view of a stamping apparatus in accordancewith one or more embodiments cut in a plane including the center line ofa work-hardening punch, illustrating the state before the work-hardeningpunch is lowered against sheet metal supported by a work-hardening die.

FIG. 7 is a cross-sectional view of a stamping apparatus of FIG. 6 cutin the same plane as FIG. 6, illustrating the state in which thework-hardening punch has lowered from the state illustrated in FIG. 6and is pressing the sheet metal.

FIG. 8 is a cross-sectional view of a stamping apparatus in accordancewith one or more embodiments cut in a plane including center line of aseparation punch, illustrating the state before the separation punch islowered against sheet metal supported by a separation die.

FIG. 9 is a cross-sectional view of a stamping apparatus of FIG. 8 cutin the same plane as FIG. 8, illustrating the state in which theseparation punch has lowered from the state illustrated in FIG. 8 and ispressing the sheet metal.

FIG. 10 is a cross-sectional view of a stamping apparatus of FIG. 8 cutin the same plane as FIG. 8, illustrating the state in which theseparation punch has further lowered from the state illustrated in FIG.9 and a workpiece part of the sheet metal has been separated by theseparation punch.

FIG. 11 is an overview of a stamping apparatus in accordance with one ormore embodiments.

DETAILED DESCRIPTION

One or more embodiments provides

a method of stamping for separating a workpiece part of sheet metal toobtain a workpiece, the method comprising:

a work-hardening step of generating work-hardening at vicinity ofoutline of the workpiece part of the sheet metal by moving awork-hardening punch having an end side facing the sheet metal towardthe sheet metal and locally pressing the vicinity of the outline alongthe outline with a work-hardening projection provided on the end side ofthe work-hardening punch; and

a separation step of punching the workpiece part of the sheet metalwhich has finished the work-hardening step with a separation punch andthereby separating the workpiece part from the sheet metal,

wherein the work-hardening projection is formed to have V-shapedcross-section with both inner wall and outer wall of the work-hardeningprojection being inclined to pressing direction of the work-hardeningpunch.

Thus, by performing the work-hardening step, work-hardening is generatedin the vicinity of the outline (hereinafter referred to as“outline-vicinity area”) in the sheet metal, making the outline-vicinityarea fragile. In addition, a dent is formed on the sheet metal by thework-hardening projection. When the workpiece part is punched in theseparation step, cracks are likely to be formed from the bottom of thedent or the vicinity thereof. As a result, an advantage is achieved inthat burrs are less likely to be formed on the separated workpiece. Inaddition, the cracks are likely to be formed in a straight way where thework-hardening is generated. As a result, an advantage is achieved inthat fractured surfaces are less likely to be formed on the separatedworkpiece. Furthermore, by forming the work-hardening projection to haveV-shaped cross-section, an advantage is achieved in that thework-hardening projection is less likely to be deformed or broken.

One or more embodiments provides

a stamping mold for separating a workpiece part of sheet metal to obtaina workpiece, the stamping mold comprising:

a work-hardening punch having an end side configured to face the sheetmetal, the end side provided with a work-hardening projection, whereinthe work-hardening projection is configured to generate work-hardeningat vicinity of outline of the workpiece part of the sheet metal bylocally pressing the vicinity of the outline along the outline;

a work-hardening die provided in pair with the work-hardening punch,wherein the work-hardening die is configured to support the sheet metalon outside of the workpiece part;

a separation punch configured to punch the workpiece part of the sheetmetal which has pressed by the work-hardening punch, and therebyseparate the workpiece part from the sheet metal; and

a separation die provided in pair with the separation punch, wherein theseparation die is configured to support the sheet metal on outside ofthe workpiece part,

wherein the work-hardening projection is formed to have V-shapedcross-section with both inner wall and outer wall of the work-hardeningprojection being inclined to pressing direction of the work-hardeningpunch.

One or more embodiments provides a stamping apparatus comprising thestamping mold described in the previous paragraph.

In one or more embodiments, the work-hardening projection is providedsuccessively on the end side of the work-hardening punch, and isconfigured to follow the outline of the workpiece part of the sheetmetal. As a result, an advantage is achieved in that work hardening islikely to be generated successively along the outline of the workpiecepart, and burrs and fracture surfaces are even less likely to be formedon the separated workpiece. Here, “the work-hardening projection isprovided successively” means that the work-hardening projection is in anannular shape or an annular shape with one cut-off portion. The term“annular shape” includes substantially circular rings, substantiallyelliptical rings, substantially polygonal rings, rings with at least onecorner of the substantially polygonal ring rounded, and other irregularshapes. In other embodiments, a plural of work-hardening projections isprovided intermittently on the end side of the work-hardening punch, andare configured to follow the outline of the workpiece part of the sheetmetal.

In one or more embodiments, inner width of the work-hardening die isconfigured to be equal to outer width of the workpiece. Meanwhile,ridgeline distance of the work-hardening projection is not limited,wherein the “ridgeline distance” refers to, if the work-hardening punchhas only one work-hardening projection, a width of an area defined by aridgeline of the work-hardening projection, and if the work-hardeningpunch has a plural of work-hardening projections, a width of an areadefined by a line connecting all the peaks of the plural ofwork-hardening projections, the same applies hereinafter. In someembodiments, the ridgeline distance of the work-hardening projection isconfigured to be substantially equal to outer width of the workpiece(which is, in some embodiments, substantially equal to inner width ofthe work-hardening die). As a result, the above-mentioned cracks aremore likely to be formed parallel to the pressing direction, making itmore likely to get a well-formed periphery of the separated workpiece.In other embodiments, peak tip(s) of the work-hardening projection(s)is/are configured to contact slightly outside the outline of theworkpiece part of the sheet metal. In other words, the ridgelinedistance of the work-hardening projection is configured to be slightlylarger than the outer width of the workpiece. In other embodiments, peaktip(s) of the work-hardening projection(s) is/are configured to contactslightly inside the outline of the workpiece part of the sheet metal. Inother words, the ridgeline distance of the work-hardening projection isconfigured to be slightly smaller than the outer width of the workpiece.

Outer width of the separation punch is not limited. Inner width of theseparation die is not limited either. In some embodiments, outer widthof the separation punch is configured to be smaller than outer width ofthe workpiece (which is, in some embodiments, substantially equal toinner width of the work-hardening die), and inner width of theseparation die is configured to be larger than outer width of theworkpiece. As a result, an advantage is achieved in that whisker burrsare less likely to be formed on the periphery of the separatedworkpiece. In other embodiments, outer width of the separation punch isconfigured to be substantially equal to the outer width of theworkpiece. In other embodiments, inner width of the separation die isconfigured to be substantially equal to the outer width of theworkpiece.

One or more embodiments further comprises a counter-punch placed insidea die opening of the work-hardening die, wherein the counter-punch isconfigured to pressure hold the workpiece part from the opposite side ofthe work-hardening punch when the sheet metal is pressed by thework-hardening punch. The counter-punch is intended to prevent warpingof the workpiece part during pressing by the work-hardening punch bypressure holding the workpiece part from the opposite side of thework-hardening punch. As a result, an advantage is achieved in that thedimensional accuracy of the separated workpiece is likely to beincreased.

One or more embodiments further comprises means for transferring thesheet metal that has pressed by the work-hardening punch from thework-hardening die to the separation die. As a result, an advantage isachieved in that the workpieces can be produced efficiently andcontinuously.

Hereinafter, one or more embodiments of a stamping apparatus, a methodof stamping, and a stamping mold are described with reference to thedrawings. In some of the drawings, x-axis, y-axis, and z-axis areillustrated. Directions of the x-axis, y-axis, and z-axis are keptconsistent in different figures. For convenience of explanation, thepositive side in the z-axis direction is sometimes referred to as“upper” side and the negative side in the z-axis direction is sometimesreferred to as “lower” side hereinafter.

FIG. 4 is a schematic perspective view of the neighborhood of a stampingmold 1 set in a stamping apparatus in accordance with one or moreembodiments. In one or more embodiments, the stamping mold 1 comprises awork-hardening section 10 and a separation section 20 as illustrated inFIG. 4. The work-hardening section 10 is designed to perform awork-hardening step in which work-hardening is generated at a certainpart of sheet metal 50. Meanwhile, the separation section 20 is designedto perform a separation step in which a workpiece P (see FIG. 10 below)is obtained by punching and separating a certain part of the sheet metal50.

After completing the work-hardening step in the work-hardening section10, the sheet metal 50 is transferred to the separation section 20,where the separation step is performed. Thus, the workpiece P isobtained by sequentially performing the work-hardening step at thework-hardening section 10 and the separation step at the separationsection 20.

Hereafter, the work-hardening section 10 and the separation section 20of the stamping mold 1 are described in more detail.

In one or more embodiments, the work-hardening section 10 comprises awork-hardening punch 11, a work-hardening die 12 and a counter-punch 30(see FIG. 6) as illustrated in FIG. 4. The work-hardening punch 11 hasan end side configured to face the sheet metal. A work-hardeningprojection 11 a is provided on the end side of the work-hardening punch11. FIG. 5 is a perspective view of the work-hardening punch 11 in thestamping apparatus illustrated in FIG. 4, viewed from the end side. Insome embodiments, the work-hardening projection 11 a is annularlyprovided at the edge on the end side of the work-hardening punch 11. Thework-hardening die 12 has a die opening 12 a to accept thework-hardening punch 11. In one or more embodiments, diameter of the dieopening 12 a (inner width W2 of the work-hardening die 12) is configuredto be substantially equal to outer width W₅ (see FIG. 10) of theworkpiece P.

FIG. 6 and FIG. 7 are cross-sectional views of a stamping apparatus inaccordance with one or more embodiments cut in a plane including thecenter line of the work-hardening punch 11. FIG. 6 illustrates the statebefore the work-hardening punch 11 is lowered against the sheet metal 50supported by the work-hardening die 12. FIG. 7 illustrates the state inwhich the work-hardening punch has lowered from the state illustrated inFIG. 6 and is pressing the sheet metal 50.

In the work-hardening section 10, the work-hardening punch 11 is loweredagainst the sheet metal 50 supported by the work-hardening die 12 asillustrated in FIG. 6 and FIG. 7. Then, as illustrated in FIG. 7, thework-hardening punch 11 locally presses the sheet metal 50 in vicinityof outline (outline-vicinity area α) of a workpiece part 51 (a part ofthe sheet metal 50 that is to be the workpiece P) along its outline withthe work-hardening projection(s) 11 a. Thus, work-hardening (aphenomenon in which hardness of metal increases due to plasticdeformation when stress is applied to it) is likely to be efficientlygenerated in the outline-vicinity area α.

In the work-hardening step of the embodiments illustrated in FIG. 6 andFIG. 7, the work-hardening punch 11 is lowered until just before the endpeak(s) of the work-hardening projection(s) 11 a reaches the top surfaceof the work-hardening die 12. When the work-hardening step is completed,an annular dent β (dent groove) is formed by the work-hardeningprojection 11 a in the outline-vicinity area α along the outline of theworkpiece part 51. At this point, the workpiece part 51 and a part(s) ofthe sheet metal 50 located outside of the workpiece part 51 (scrappart(s) 52) are connected only by thin sheet-like part(s) in theoutline-vicinity area α. Although hardened by the work-hardeningdescribed above, this sheet-like part(s) is/are very easy to get cracksand fragile due to its hardness and thinness.

During the work-hardening step, the workpiece part 51 of the sheet metal50 comes down into the die opening 12 a of the work-hardening die 12 asillustrated in FIG. 7 as it is pressed downward (toward the die opening12 a) by the end side of the work-hardening punch 11. At this point, theoutline-vicinity area α of the workpiece part 51 receives upwardresistance force (tensile force) from the scrap part(s) 52 of the sheetmetal 50. In addition, the outer periphery of the workpiece part 51,which has come down into the die opening 12 a, receives upwardresistance force (frictional force) from the inner periphery of thework-hardening die 12 (the inner wall of the die opening 12 a). Due tothese forces, the workpiece part 51 is likely to warp convexly to thelower direction during the work-hardening step.

In this regard, one or more embodiments is provided with a counter-punch30 placed inside the die opening 12 a of the work-hardening die 12. Thecounter-punch 30 is biased upward (to the work-hardening punch 11 side)by biasing means 31. In some embodiments, the biasing means 31 is a coilspring. The counter-punch 30 pressure holds the workpiece part 51 upwardwhen the workpiece part 51 is pressed downward by the work-hardeningpunch 11. In other words, the workpiece part 51 is sandwiched betweenthe work-hardening punch 11 and the counter-punch 30. Thus, theworkpiece part 51 is more likely to be kept in a substantially flatstate during the work-hardening step, and less likely to get warped. Asa result, an advantage is achieved in that the dimensional accuracy ofthe obtained workpiece P is likely to be increased. The counter-punch 30also functions as means for removing the sheet metal 50 from thework-hardening die 12, which is a so-called knockout.

The cross-sectional shape of the work-hardening projection(s) 11 a ofthe work-hardening punch 11 (in some embodiments, cross-sectional shapein the plane including the center line of the work-hardening punch 11)is V-shaped, as illustrated in FIG. 6. In other words, the inner wall 11a ₁ (wall facing the center side of the work-hardening punch 11) and theouter wall 11 a ₂ (wall facing the periphery side of the work-hardeningpunch 11) of the work-hardening projection(s) 11 a are inclined withrespect to pressing direction (direction in which the work-hardeningpunch 11 is lowered) so that the distance between them increases fromthe end side (negative side in the z-axis direction) to the base side(positive side in the z-axis direction).

As a result, an advantage is achieved in that the work-hardeningprojection(s) 11 a become more strong and less likely to be deformed orbroken. This advantage is more helpful in embodiments in which thework-hardening projection(s) 11 a is/are very fine structure(s) with itsheight H₁ ranging from about 0.5 mm to 3 mm. The height H₁ of thework-hardening projection(s) 11 a is determined according to thethickness of the sheet metal 50 and/or other factors, and is notparticularly limited. In some embodiments, as illustrated in FIG. 6,inclination angle θ1 of the inner wall 11 a ₁ is substantially the sameas inclination angle θ2 of the outer wall 11 a ₂. As a result, when thework-hardening projection(s) 11 a is/are pressed into the sheet metal50, the outward component of the force that the inner wall 11 a ₁receives from the sheet metal 50 becomes substantially equal to theinward component of the force that the outer wall 11 a ₂ receives fromthe sheet metal 50, making the work-hardening projection(s) even lesslikely to be deformed or broken.

The inclination angle θ1 of the inner wall 11 a ₁ and the inclinationangle θ2 of the outer wall 11 a ₂ are not particularly limited as longas they are larger than 0° and smaller than 90°. In one or moreembodiments, at least one of the inclination angle θ1 or θ2 is more than10°. In some embodiments, at least one of the inclination angle θ1 or θ2is more than 15°. In at least one embodiment, at least one of theinclination angle θ1 or θ2 is more than 20°. The larger the inclinationangle θ1 or θ2 is, the easier it is to maintain strength of thework-hardening projection(s) 11 a. In one or more embodiments, at leastone of the inclination angle θ1 or θ2 is less than 60°. In someembodiments, at least one of the inclination angle θ1 or θ2 is less than50°. In at least one embodiment, at least one of the inclination angleθ1 or θ2 is less than 45°. The smaller the inclination angle θ1 or θ2is, the easier it is for the work-hardening projection(s) 11 a to getinto the sheet metal 50.

The ridgeline distance W₁ (see FIG. 6) of the work-hardening projection11 a is not particularly limited. In some embodiments, the ridgelinedistance W₁ is configured to be substantially equal to inner width W₂ ofthe work-hardening die 12. In other words, the ridgeline of thework-hardening projection(s) 11 a is positioned directly above the innersurface of the work-hardening die 12. As a result, in the separationstep described below, cracks C, which are formed starting from near thebottom of the dent β, are more likely to be formed parallel to thepressing direction (z-axis direction) as illustrated in FIG. 9, makingit more likely to get a well-formed periphery of the separated workpieceP. In other embodiments, the ridgeline distance W₁ is configured to beslightly larger than inner width W₂ of the work-hardening die 12. Inother embodiments, the ridgeline distance W₁ is configured to beslightly smaller than inner width W₂ of the work-hardening die 12.

When the work-hardening step in the work-hardening section 10 iscompleted, the work-hardening punch 11 is raised, then the sheet metal50 is transferred from the work-hardening section 10 to the separationsection 20, where the separation step is performed.

In one or more embodiments, the separation section 20 comprises aseparation punch 21 and a separation die 22 as illustrated in FIG. 4. Insome embodiments, the separation punch 21 has a flat end side. Theseparation die 22 has a die opening 22 a to accept the separation punch21.

FIG. 8-10 are cross-sectional views of a stamping apparatus inaccordance with one or more embodiments cut in a plane including centerline of the separation punch 21. FIG. 8 illustrates the state before theseparation punch 21 is lowered against the sheet metal 50 supported bythe separation die 22. FIG. 9 illustrates the state in which theseparation punch 21 has lowered from the state illustrated in FIG. 8 andis pressing the sheet metal 50. FIG. 10 illustrates the state in whichthe separation punch 21 has further lowered from the state illustratedin FIG. 9 and a workpiece part 51 of the sheet metal 50 has beenseparated by the separation punch 21.

In the separation section 20, the separation punch 21 is lowered towardthe sheet metal 50 supported by the separation die 22 as illustrated inFIG. 8-10. The workpiece part 51 of the sheet metal 50, in which workhardening has generated in the outline-vicinity area α in thework-hardening step, is pressed to the die 22 side (lower side) by theseparation punch 21, and is separated from the scrap part(s) 52 asillustrated in FIG. 10. The separated workpiece part 51 (workpiece P)falls into the die opening 22 a. The workpiece P is used either as afinal product or a intermediate product.

As already described, work-hardening has been generated and sheet-likepart(s) have been formed in the outline-vicinity area α in thework-hardening step. Therefore, when the separation punch 21 presses theworkpiece part 51 in the separation step, cracks C starting at thebottom of the dent β are likely to be formed in the most fragile part inthe outline-vicinity area α (that is, the thinnest part) as illustratedin FIG. 9. As the dent β is formed in a cross-sectional V-shapeaccording to the shape of the work-hardening projection(s) 11 a, thecracks C are likely to be formed straight down from the bottom of thedent 1. As a result, the part of the sheet metal 50 where the cracks Care formed is likely to be smooth burnished surface rather than roughfractured surface, making it more likely to get a smoother periphery ofthe separated workpiece P with less fracture surface.

Moreover, because of the dent β formed on the upper side of theoutline-vicinity area α, and of the smooth cracks C described above,burrs are less likely to be formed on upper end of the periphery of theseparated workpiece P. As a result, an advantage is achieved in that thepost-process of removing burrs from the workpiece P can be eliminated,and the workpiece P can be produced continuously and efficiently.

In some embodiments, outer width W₃ of the separation punch 21 isconfigured to be smaller than outer width W₅ of the workpiece P, andinner width W₄ of the separation die 22 is configured to be larger thanouter width W₅ of the workpiece P. As a result, an advantage is achievedin that whisker burrs are less likely to be formed on the periphery ofthe separated workpiece P. In other embodiments, at least one of theouter widths W₃₃ or a diameter of the die opening 22 a (that is, theinner width W₄) is configured to be substantially equal to the outerwidth W₅ (see FIG. 10) of the workpiece P.

FIG. 11 is an overview of a stamping apparatus in accordance with one ormore embodiments. In one or more embodiments, the sheet metal 50 isautomatically transferred from the work-hardening section 10 to theseparation section 20 by a sheet metal transfer means. In someembodiments, the stamping apparatus is a progressive stamping apparatuscomprising an uncoiler 91, a feeder 92 as the sheet metal transfermeans, and a stamping press 93, as illustrated in FIG. 11. In this case,the stamping mold 1 is attached to the stamping press 93. In at leastone embodiment, the feeder 92 is a roll feeder that feeds the sheetmetal 50 with rotating rollers. In at least one other embodiment, thefeeder 92 is a gripper feeder that feeds the sheet metal by gripping thesheet metal with grippers. In other embodiments, the stamping apparatusis a transfer press apparatus comprising fingers, which hold the sheetmetal 50 and/or the workpiece P, as the sheet metal transfer means. Inother embodiments, the sheet metal 50 is manually transferred from thework-hardening section 10 to the separation section 20.

As described so far, the stamping apparatus of one or more embodimentsmakes it possible to obtain a workpiece P with less burrs and fracturedsurfaces, while having a relatively simple structure and being easy tomaintain and operate. In the embodiments illustrated in FIG. 4-10, thework-hardening punch 11 and the removal punch 21 are both substantiallycylindrical, as they are intended to separate the substantiallydisk-shaped workpiece P. In other words, the work-hardening punch 11 andthe removal punch 21 have a substantially circular cross-sectional shapewhen cut in a plane perpendicular to the pressing direction. In otherembodiments, cross-sectional shape (when cut in a plane perpendicular tothe pressing direction) of at least one of the work-hardening punch 11or the separation punch 21 is, for example, but not limited to, asubstantially oval shape, a substantially polygonal shape, asubstantially polygonal shape with at least one corner rounded, or anyother irregular shapes, according to shape of the workpiece P.

1. A stamping apparatus for separating a workpiece part of sheet metalto obtain a workpiece, the stamping apparatus comprising: awork-hardening punch having an end side configured to face the sheetmetal, the end side provided with a work-hardening projection, whereinthe work-hardening projection is configured to generate work-hardeningat vicinity of outline of the workpiece part of the sheet metal bylocally pressing the vicinity of the outline along the outline; awork-hardening die provided in pair with the work-hardening punch,wherein the work-hardening die is configured to support the sheet metalon outside of the workpiece part; a separation punch configured to punchthe workpiece part of the sheet metal which has pressed by thework-hardening punch, and thereby separate the workpiece part from thesheet metal; and a separation die provided in pair with the separationpunch, wherein the separation die is configured to support the sheetmetal on outside of the workpiece part, wherein the work-hardeningprojection is formed to have V-shaped cross-section with both inner walland outer wall of the work-hardening projection being inclined topressing direction of the work-hardening punch.
 2. The stampingapparatus of claim 1, wherein the work-hardening projection is providedsuccessively on the end side of the work-hardening punch, and isconfigured to follow the outline of the workpiece part of the sheetmetal.
 3. The stamping apparatus of claim 1, wherein ridgeline distanceof the work-hardening projection and inner width of the work-hardeningdie are both configured to be substantially equal to outer width of theworkpiece, outer width of the separation punch is configured to besmaller than outer width of the workpiece, and inner width of theseparation die is configured to be larger than outer width of theworkpiece.
 4. The stamping apparatus of claim 1, further comprising: acounter-punch placed inside a die opening of the work-hardening die,wherein the counter-punch is configured to pressure hold the workpiecepart from the opposite side of the work-hardening punch when the sheetmetal is pressed by the work-hardening punch.
 5. The stamping apparatusof claim 1, further comprising: means for transferring the sheet metalthat has pressed by the work-hardening punch from the work-hardening dieto the separation die.
 6. A method of stamping for separating aworkpiece part of sheet metal to obtain a workpiece, the methodcomprising: a work-hardening step of generating work-hardening atvicinity of outline of the workpiece part of the sheet metal by moving awork-hardening punch having an end side facing the sheet metal towardthe sheet metal and locally pressing the vicinity of the outline alongthe outline with a work-hardening projection provided on the end side ofthe work-hardening punch; and a separation step of punching theworkpiece part of the sheet metal which has finished the work-hardeningstep with a separation punch and thereby separating the workpiece partfrom the sheet metal, wherein the work-hardening projection is formed tohave V-shaped cross-section with both inner wall and outer wall of thework-hardening projection being inclined to pressing direction of thework-hardening punch.
 7. A stamping mold for separating a workpiece partof sheet metal to obtain a workpiece, the stamping mold comprising: awork-hardening punch having an end side configured to face the sheetmetal, the end side provided with a work-hardening projection, whereinthe work-hardening projection is configured to generate work-hardeningat vicinity of outline of the workpiece part of the sheet metal bylocally pressing the vicinity of the outline along the outline; awork-hardening die provided in pair with the work-hardening punch,wherein the work-hardening die is configured to support the sheet metalon outside of the workpiece part; a separation punch configured to punchthe workpiece part of the sheet metal which has pressed by thework-hardening punch, and thereby separate the workpiece part from thesheet metal; and a separation die provided in pair with the separationpunch, wherein the separation die is configured to support the sheetmetal on outside of the workpiece part, wherein the work-hardeningprojection is formed to have V-shaped cross-section with both inner walland outer wall of the work-hardening projection being inclined topressing direction of the work-hardening punch.